Add subscription protocol
Add a way to subscribe to different SignalK messages by specifying stream=subscribe
in the query string and then by sending a JSON command message the consumer can
specify which SignalK paths it is interested. Current implementatin works only
with deltas and ignores context, just considers the paths in the update message.
See also https://github.com/SignalK/maptracker/blob/5cd3ba626188563640329253d6792aeee41a46fc/index.html#L158
Merge pull request #12 from SignalK/subscription-protocol

Add subscription protocol